The invention relates to a method for shifting a doctor blade, that is set against a cylinder of a printing machine, in direction of the longitudinal axis of the doctor blade, as well as an apparatus for carrying out this method.
In printing machines, doctor blade assemblies are generally used for doctoring the surface of a rotating cylinder, e.g. the printing cylinder, by means of a doctor blade. In a gravure printing machine, for example, such a doctor device has the function to wipe the surface of t he gravure printing cylinder after it has been inked and before the proper print process takes place, so that printing ink will remain only in the printing, engraved portions of the printing cylinder surface.
In conventional printing machines, the doctor blades are generally formed by thin sheets of hardened or non-hardened spring steel. An edge of a doctor blade that is set against the surface of the print cylinder or anilox roller, is subject to considerable wear. In order to increase the lifetime of the doctor blade, the doctor blade is oscillated in its longitudinal direction, so that the wear of the doctor blade is distributed as far as possible. The frequency of such an oscillation may for example be 1 Hz or more, depending on the production speed, and the oscillation stroke may for example be as large as 40 mm. Although such an oscillation of the doctor blade distributes the locus of wear, it is still necessary relatively frequently to replace a worn doctor blade. This necessitates a stop of the printing machine.
In order to further reduce the wear of the doctor blade, doctor blades have been used, which have a ceramic coating on the side facing the printing cylinder. This increases the lifetime of the doctor blade to a multiple.
Applicant's European patent application EP 1 362 696 suggests to oscillate the doctor blade with a frequency in the acoustic or ultrasonic range. This measure serves to remove contaminations from the gravure printing cylinder. Here, the stroke of the vibration movement may be less than 0.2 mm.
On the one hand, a lateral oscillation of the doctor blade is desirable in order to avoid an uneven wear of the doctor blade, but on the other hand the oscillation may have a negative effect on the printed image, because differences in the speed of the doctor blade may result in slightly different amounts of ink in the engraved portions of the printing cylinder. With the result that ink is non-uniformly applied onto the medium. Especially the difference between a range where the doctor blade moves with an even speed, and a range where the direction of movement of the doctor blade is reversed, may be visible in the printed image. Since the frequency of the oscillation of the doctor blade is in many cases coupled to the rotary frequency of the printing cylinder, different motion patterns are used for different doctor blades, in quality multi colour printing, for example, in order to prevent the deviations in the individual colours from adding-up. However, in multi colour printing for particularly high quality, the different speeds, directions of movement and direction reversals accompanying the oscillation of the doctor blade may still have a negative effect on the printed image.